Grover's algorithm lets you search an unsorted list of four elements with just one "is this thing in the list?" query. Classically, of course, it requires four queries.

More precisely, given f: 2^n -> {0,1} which is guaranteed to hit 1 exactly once, Grover finds the one input which hits 1, and it does so using about 2^{n/2} queries of f; but the constants happen to line up so that when n=2, exactly one query is required.